This invention relates to a method and a formulation for orally administering a bioactive agent encapsulated in one or more biocompatible polymer or copolymer excipients, preferably a biodegradable polymer or copolymer, affording microcapsules which due to their proper size and physical chemical properties results in the microcapsules and contained agent reaching and being effectively taken up by the folliculi lymphatic aggregati, otherwise known as the "Peyer's patches", of the gastrointestinal tract in an animal without loss of effectiveness due to the agent having passed through the gastrointestinal tract. Similar folliculi lymphatic aggregati can be found in the respiratory tract, genitourinary tract, large intestine and other mucosal tissues of the body such as ophthalmic tissues. Hereafter, the above-described tissues are referred to in general as mucosally-associated lymphoid tissues.
The use of microencapsulation to protect sensitive bioactive agents from degradation has become well-known. Typically, a bioactive agent is encapsulated within any of a number of protective wall materials, usually polymeric in nature. The agent to be encapsulated can be coated with a single wall of polymeric material (microcapsules), or can be homogeneously dispersed within a polymeric matrix (microspheres). (Hereafter, the term microcapsules refers to both microcapsules and microspheres). The amount of agent inside the microcapsule can be varied as desired, ranging from either a small amount to as high as 95% or more of the microcapsule composition. The diameter of the microcapsule can also be varied as desired, ranging from less than one micrometer to as large as three millimeters or more.
Peyer's patches are aggregates of lymphoid nodules located in the wall of the small intestine, large intestine and appendix and are an important part of body's defense against the adherence and penetration of infectious agents and other substances foreign to the body. Antigens are substances that induce the antibody-producing and/or cell-mediated immune systems of the body, and include such things as foreign protein or tissue. The immunologic response induced by the interaction of an antigen with the immune system may be either positive or negative with respect to the body's ability to mount an antibody or cell-mediated immune response to a subsequent reexposure to the antigen. Cell-mediated immune responses include responses such as the killing of foreign cells or tissues, "cell-mediated cytoxicity", and delayed-type hypersensitivity reactions. Antibodies belong to a class of proteins called immunoglobulins (Ig), which are produced in response to an antigen, and which combine specifically with the antigen. When an antibody and antigen combine, they form a complex. This complex may aid in the clearance of the antigen from the body, facilitate the killing of living antigens such as infectious agents and foreign tissues or cancers, and neutralize the activity of toxins or enzymes. In the case of the mucosal surfaces of the body the major class of antibody present in the secretions which bathe these sites is secretory immunoglobulin A (sIgA). Secretory IgA antibodies prevent the adherence and penetration of infectious agents and other antigens to and through the mucosal tissues of the body.
While numerous antigens enter the body through the mucosal tissues, commonly employed immunization methods, such as intramuscular or subcutaneous injection of antigens or vaccines, rarely induce the appearance of sIgA antibodies in mucosal secretions. Secretory IgA antibodies are most effectively induced through direct immunization of the mucosally-associated lymphoid tissues, of which the Peyer's patches of the gastrointestinal tract represent the largest mass in the body.
Peyer's patches possess IgA precursor B cells which can populate the lamina propria regions of the gastrointestinal and upper respiratory tracts and differentiate into mature IgA synthesizing plasma cells. It is these plasma cells which actually secrete the antibody molecules. Studies by Heremans and Bazin measuring the development of IgA responses in mice orally immunized with antigen showed that a sequential appearance of antigen-specific IgA plasma cells occurred, first in mesenteric lymph nodes, later in the spleen, and finally in the lamina propria of the gastrointestinal tract (Bazin, H., Levi, G., and Doria, G. Predominant contribution of IgA antibody-forming cells to an immune response detected in extraintestinal lymphoid tissues of germ free mice exposed to antigen via the oral route. J. Immunol. 105:1049; 1970 and Crabbe, P. A., Nash, D. R., Bazin, H., Eyssen, H. and Heremans, J. F. Antibodies of the IgA type in intestinal plasma cells of germ-free mice after oral or parenteral immunization with ferritin. J. Exp. Med. 130:723; 1969). Subsequent studies have shown that oral administration of antigens leads to the production of sIgA antibodies in the gut and also in mucosal secretions distant to the gut, e.g., in bronchial washings, colostrum, milk, saliva and tears (Mestecky, J., McGhee, J. R., Arnold, R. R., Michalek, S. M., Prince, S. J. and Babb, J. L. Selective induction of an immune response in human external secretions by ingestion of bacterial antigen. J. Clin. Invest. 61:731; 1978, Montgomery, P. C., Rosner, B. R. and Cohen, J. The secretory antibody response. Anti-DNP antibodies induced by dinitrophenylated Type III pneumococcus. Immunol. Commun. 3:143; 1974, and Hanson, L. A., Ahistedt, S., Carlsson, B., Kaijser, B., Larsson, P., MattsbyBaltzer, A., Sohl Akerlund, A., Svanborg Eden, C. and Dvennerholm, A. M. Secretory IgA antibodies to enterobacterial virulence antigens: their induction and possible relevance, Adv. Exp. Med. Biol. 1007:165; 1978). It is apparent, therefore, that Peyer's patches are an enriched source of precursor IgA cells, which, subsequent to antigen sensitization, follow a circular migrational pathway and account for the expression of IgA at both the region of initial antigen exposure and at distant mucosal surfaces. This circular pattern provides a mucosal immune system by continually transporting sensitized B cells to mucosal sites for responses to gut-encountered environmental antigens and potential pathogens.
Of particular importance to the present invention is the ability of oral immunization to induce protective antibodies. It is known that the ingestion of antigens by animals results in the appearance of antigen-specific sIgA antibodies in bronchial and nasal washings. For example, studies with human volunteers show that oral administration of influenza vaccine is effective at inducing secretory anti-influenza antibodies in nasal secretions.
Extensive studies have demonstrated the feasibility of oral immunization to induce the common mucosal immune system, but with rare exception the large doses require to achieve effective immunization have made this approach impractical. It is apparent that any method or formulation involving oral administration of an ingredient be of such design that will protect the agent from degradation during its passage through the gastrointestinal tract and target the delivery of the ingredient to the Peyer's patches. If not, the ingredient will reach the Peyer's patches, if at all, in an inadequate quantity or ineffective condition.
Therefore, there exists a need for a method of oral immunization which will effectively stimulate the immune system and overcome the problem of degradation of the antigen during its passage through the gastrointestinal tract to the Peyer's patch. There exists a more particular need for a method of targeting an antigen to the Peyer's patches and releasing that antigen once inside the body. There also exists a need for a method to immunize through other mucosal tissues of the body which overcomes the problems of degradation of the antigen and targets the delivery to the mucosally-associated lymphoid tissues. In addition, the need exists for the protection from degradation of mucosally applied bioactive agents, improves and/or targets their entrance into the body through the mucosally-associated lymphoid tissues and releases the bioactive agent once it has entered the body.